Background It is still not clear whether a low level of education increases the risk of developing Alzheimer's disease (AD).

Two common problems in cohort studies involving an elderly population and a two-step diagnostic procedure are the loss to follow-up without data on the presence of AD, and the fact that, in general, people with higher levels of education perform better on traditional cognitive tests, such as the Mini-Mental State Examination (MMSE).

Both phenomena may lead to misclassification, resulting in a biased association between level of education and AD.

This study investigated to what extent these selection mechanisms may influence this association.

Methods In the community-based Amsterdam Study of the Elderly (AMSTEL) a cohort at risk for AD was selected of 3778 people aged 65-84 years.

Level of education was expressed in two categories : low (primary education or less) versus high (partial secondary education to completed university education).

At follow-up, a sub-sample of elderly people was selected for further diagnostic evaluation, using a memory test in addition to the MMSE.

Clinical diagnoses of AD were made according to DSM-IV (Diagnostic and Statistical Manual of Mental Disorders) criteria.

To examine the extent to which loss to follow-up may have affected the results, a sensitivity analysis was performed comparing two extreme possibilities. (...)